The invention relates to a magnetic field sensor, in particular for measurement of the rotation speed of a rotating transmission component, having a Hall probe having at least one permanent magnet, with the Hall probe being located in the area of influence of the magnetic field of the permanent magnet, having a housing which surrounds the Hall probe, wherein the permanent magnet is attached to the housing, wherein the housing is at least partially composed of plastic and the permanent magnet is in the form of a ring or hollow cylinder, wherein the longitudinal axis of the permanent magnet and an induction axis of the Hall sensor essentially coincide, with the induction axis being oriented at right angles to the electrical field direction of the Hall voltage and to the electrical main current direction in the Hall probe.
Magnetic field sensors of the type mentioned above are preferably used for rotation speed monitoring in goods vehicles, because the signal transmitters, in which field plates or Hall generators are used as active elements, have the advantage over induction transmitters that it is possible to detect relatively low rotation speeds or movements in the stationary range. In addition to being used for rotation speed measurement in a motor vehicle transmission, magnetic field sensors according to the invention can also be used to monitor other movements, for example rotary movements or translations.
A magnetic field sensor of a conventional type is already known from EP 0 546 355 B1. The extraordinarily accurate operation of this component can be achieved only on the basis of a highly complex manufacturing and assembly process. The large number of work steps include, inter alia, the attachment of the permanent magnet to a holder which has a push fit with the permanent magnet, by means of a UV-curing adhesive following previous accurate adjustment of the position of the Hall probe with respect to the permanent magnet. The normally extraordinary operating conditions of these magnetic field sensors, in particular with regard to considerable temperature fluctuations from −40° C. to +150° C. and with regard to mechanical vibration and shock loads, require the permanent magnet to be mounted in the magnetic field sensor with extraordinary robustness. Adequate strength of the joint between the heavy permanent magnet and the holder which is provided for this purpose can be achieved, for example, by the contact points being activated by means of a primer, before the application of the UV-curing adhesive. After the UV-curing, additional thermal curing must be carried out for at least three hours at room temperature in order to make it possible to ensure that the joint is of the necessary quality. In order to ensure that the pulse width of the signals which are generated by the Hall probe as it passes over a gear wheel or a similar magnetic field concentrator is as desired, the Hall probe, which is normally in the form of a Hall IC, must be aligned exactly with respect to the permanent magnet, which is preferably in the form of a ring magnet. The disadvantage of the large amount of effort for assembly and manufacture of conventional magnetic field sensors is evident from the large number of work steps, only some of which have been mentioned here, but which must be carried out with very high precision.
Against the background of the problems and disadvantages of the prior art, the invention is based on the object of providing a magnetic field sensor which can be produced with less effort and which is less susceptible to defects.
A magnetic field transmitter of the type mentioned in the introduction is already known from German Utility Model DE 295 16 373 U1, in which an annular permanent magnet is inserted into a sleeve-like housing such that it is aligned, and the Hall probe is then adjusted in a complex manner in particular in the direction of the magnetic axis of the permanent magnet, and is finally fixed to the permanent magnet and to the housing by means of an encapsulation compound.
The International Patent Application WO 03/040659 A1, U.S. Pat. 6,291,990 B1 and German Laid-Open Specification DE 101 46 157 A1 each disclose arrangements with a sensor, in which the Hall probe of a sensor is arranged off a magnetic null axis, which requires the sensor to be positioned very accurately in some way with respect to a rotating component which measurably varies the magnetic flux in the area of the Hall probe of the sensor, as a significant change. The signal from a sensor arrangement such as this must furthermore be of special quality, since the arrangement of the probe in the area which is magnetically not neutral results in a permanent basic signal whose magnitude is highly dependent on the magnetic properties of the area surrounding the sensor, and the signal which is actually to be evaluated is highly dependent on the distance between the sensor and the actual pulse-transmitting marks.